An organic electroluminescence device (OLED) has a basic structure including an anode layer, a cathode layer, and a light-emitting layer, which is one or more organic layer, sandwiched between the anode layer and the cathode layer. Upon application of a voltage, electrons and holes are injected into the organic layer from the anode and the cathode, respectively, and then the electrons and holes migrate in the light-emitting layer until they meet and recombine to generate excitons whose energy decays in the form of light, and thus light radiates.
An OLED generally includes a first electrode layer (i.e., an anode layer or a cathode layer), a second electrode layer (i.e., a cathode layer or an anode layer), and a light-emitting layer disposed between the first and second electrode layers; a planarization layer and a TFT (Thin Film Transistor) are disposed on a light-exiting side of the first electrode layer. Here, an OLED in which light is emitted from a side, which faces the planarization layer, of the first electrode is a bottom emission OLED, and the first electrode should be a transparent (such as ITO, i.e., Indium Tin Oxide) electrode layer, while the second electrode layer is generally an opaque electrode layer. Considering the structure of a bottom emission OLED, the bottom emission OLED generally has a relatively low aperture ratio, because some light is blocked by TFTs. To achieve a practically usable luminance, a voltage may be improved to increase the luminance of the bottom emission OLED, which, however, has a negative effect on the service life of the device and the material. Therefore, for a bottom emission OLED, demands on its performance indexes, such as service life of its material, light-extraction efficiency and the like, are higher.
In addition, energy loss occurs during the light-emitting process of an OLED, which is mainly reflected in the following two aspects.
Firstly, when injected carriers recombine to emit light in the light-emitting layer, not all injected energy is converted into photons, and part of exciton energy is consumed in non-radiative transition processes such as lattice vibrations, deep level impurity transitions and the like, and this phenomenon may be described by using inner quantum efficiency.
Secondly, due to total reflection occurring at the interfaces between the anode layer and the substrate, between the substrate and the air, and the like of the OLED, guided waves occurring at the interface of the anode layer and the light-emitting layer of the OLED, and surface plasmon loss near the metal electrode, after the light emitted from the light-emitting layer passes through the above multi-layer structure, only about 20% of the light emitted from the OLED can be emitted into the air, while about 80% of the light is consumed, and this phenomenon may be described by using outer quantum efficiency, which represents the efficiency of extracting light from the OLED, i.e., light extraction efficiency.
At present, an OLED with an inner quantum efficiency of nearly 100% can be realized in theory by improving performance of materials; however, kind of such materials is very limited. On the other hand, light extraction efficiency of an OLED may be greatly improved by using various techniques, for example, a technique of forming a surface microstructure on the transparent electrode layer to reduce the loss caused by guided wave, a technique of attaching photonic crystals or a microlens array to a glass substrate to reduce the total internal reflection, a technique of manufacturing a corrugated cathode to reduce its surface plasmon loss, and a technique of using an optical microcavity structure. However, in techniques of using photonic crystals, a periodic or quasi-periodic microstructure formed on the cathode, and the like, a nano photocopy technique is generally adopted, which has a complicated manufacturing process. Also, microcavity effect is likely to result in chromatic aberration, narrower viewing angle, and other drawbacks.